You simply genetically modify the cereal crops so that they cause sterility in the human male.
Sterility in the human male is already occuring.
Google sperm count and you will find that Western men are suffering an infertility crisis. Professor Niels Skakkebaek of the University of Copenhagen presented data to the WHO indicating sperm counts had fallen by about a half over the past 50 years.
The human population explosion may eventually be self limiting. I suppose it would be surprising if chemicals that affect wildlife reproduction had no impact on human reproduction.